I have had mushroom type fungus in my greenhouse soil, - I grew crops straight into the ground, it did not seem to affect any plants that I grew last year, but should I try and use some sort of soil steriliser to ensure this does not happen again.?

It's unlikely the fungus will effect your vegetable crops. It's probably living on some organic matter that is decomposing under the soil - could even be compost you've dug in. Don't worry - it's how nature breaks nutrients down and makes it available for your plants to take up. Love thy Fungus ;-)